The present invention relates to an optical filter formed with birefringent plates, and more particularly relates to a spatial frequency filter for eliminating an alias involved in a two-dimensional solid-state image sensor for a color TV camera.
A two-dimensional solid-state image sensor is widely utilized in various devices such as a TV camera, a fiber scope, etc. Video information is obtained from the image sensor by discrete sampling of spatial information of an optical image. A frequency of the sampling depends on the configuration of an optical system and on the arrangement of apertures of pixels of the image sensor. Such an image sensor is provided with a spatial frequency filter (or optical low pass filter). The modulation transfer function (MTF) of the image sensor is restricted by a Nyquist limit (or Nyquist frequency) which is intrinsic to the individual image sensor. In addition, video information of a high frequency exceeding the Nyquist limit is folded back to the frequency area below the Nyquist limit. This folding back is called "alias" which induces spurious video information and thus degrades the quality of the obtained video information.
In a color TV camera using a single-plate solid-state image sensor, in order to obtain color signals incident light is spatially modulated. Signals obtained by this spatial modulation are processed in a known manner. At this time, information of the incident light having a frequency equal to the spatial modulation frequency involves a color spurious signal.
From this, a spatial frequency filter having a trap corresponding to the spatial modulation frequency is required to reduce the color spurious signal. In this case, however, if the spatial frequency filter has a sharp cutting-off characteristic, a certain amount of residual color spurious signals remains around the trap.
To obtain color signals of three primary colors, it is necessary to produce not only a base band signal for luminance information but also two modulated color signals. These color modulation signals are produced by proper processing of multiplex signals which are obtained by means of a color mosaic filter. In such a color mosaic filter, specific pixels for one color information are arranged in a striped pattern, while other pixels for the other color information are arranged in a checkered pattern. In this case, a color spurious signal due to the striped pixel pattern is caused by the signal having a horizontal spatial frequency of the two dimensional spatial frequency plane, and a color spurious signal due to the checkered pattern is caused by the signal having a slant spatial frequency thereof.
A conventional approach to eliminate the influence of the alias is disclosed in Japanese Patent Disclosure No. 59-279. Assume that Px defines the pixel pitch in the horizontal axis of a two-dimensional solid-state image sensor and Py defines the pixel pitch in the vertical axis thereof. A spatial frequency filter of the above Japanese Patent Disclosure is made of two optical filters, thereby forming traps (null points of MTF=0) of the spatial frequency filter on a line passing through the points (Px/2, Py/2) and (-Px/2, Py/2), on a line passing through the points (Px/2, -Py/2) and (-Px/2, -Py/2), on a line passing through the points (Px/2, Py/2) and (Px/2, -Py/2), and on a line passing through the points (-Px/2, Py/2) and (-Px/2, -Py/2). (In this case, the trap lines form a rectangular figure.) Further, this Japanese Patent Disclosure shows another spatial frequency filter by which traps are formed on slant lines respectively passing through the points (Px/2, Py/2), (-Px/2, Py/2), (-Px/2, -Py/2) and (Px/2, -Py/2). (In this case, the trap lines form a rhombic figure.)
When the spatial frequency filter with the rectangular trap lines is employed, unnecessary video information exceeding the Nyquist limit, to which belongs the slant axis of the X-Y two-dimensional plane of the image sensor, cannot be sufficiently suppressed. Such unnecessary video information could invite spurious signals when a spatial modulation is effected along the slant axis. On the other hand, when the spatial frequency filter with the rhombic trap lines is employed, unnecessary video information of the slant axis can be suppressed. In this case, however, the degree of suppression of spurious video information exceeding the Nyquist limit, to which the horizontal (or vertical) axis of the X-Y image sensor plane belong, is not yet sufficient. According to the prior art spatial frequency filter, the requirement for suppressing spurious information of the slant axis is, in certain cases, contradictory to that for suppressing spurious information of the horizontal or vertical axis.
The above potential contradiction is associated with a material problem in a color TV camera using a solid-state image sensor. According to the prior art spatial frequency filter, except for four cross-points of the four trap lines, the MTF has the absolute value of the product of a cosine function having a given frequency and another cosine function having another given frequency (cf. solid lines in FIGS. 3 to 5). From this, even though the signal reduction rate just on the trap lines is sufficient (e.g., points at 1/Py, -1/Py in FIG. 3), the signal reduction rate near the respective trap lines is not sufficient. Therefore, when the prior art spatial frequency filter is employed for a color TV camera, color spurious information which slightly avoids the trap lines is involved in a color video signal, resulting in the appearance of a spurious color image on a color display.